This invention relates in general to the display of electrocardiograph (ECG) signals and more specifically to apparatus and methods for displaying such signals accumulated over a substantial monitoring time. Even more specifically, this invention is directed to methods and apparatus for presenting large volumes of ECG data in a compact visual form.
The fundamental problem associated with the long term ECG monitoring of a patient is that a large volume of data, once taken from the patient, is not readily usable by a cardiologist. This data is often collected on a roll or a cassette of magnetic tape and requires special equipment for playback. Even if the special equipment is available, it is still inconvenient for the cardiologist to examine the data quickly.
For this reason, systems for displaying data taken over an 8-10 hour period in a compact and easily readable form are highly advantageous. U.S. Pat. Nos. 3,893,453 and 3,951,135 issued to Goldberg et al. on July 8, 1975 and Apr. 20, 1976 respectively, teach a rotating drum system for recording a large volume of ECG information. These patents generally recognize the fundamental problem presented to a cardiologist attempting to analyze ECG signal data obtained over a long monitoring period. The Goldberg et al solution to this display problem is to record the continuous ECG signal in a helical fashion about a cylindrical record member which is then slit and/or opened along a line to present a flat picture of the collected ECG signals. In the Goldberg system, the speed of the rotating drum is varied in proportion to the patient's heart rate. Thus, all heart beats, as recorded on the drum, appear to have the same horizontal span. There is no discernable distinction between a row of ECG pulses responsive to a heart rate of a 120 beats per minute and a row of pulses responsive to a heart rate of 80 beats per minute. In order to interpret pulse rate information in the Goldberg et al system, some sort of time base marker would have to be recorded along with the ECG signals as an interpretation aid. In addition, at a high pulse rate the Goldberg type system utilizes film at a high rate. Thus, contrary to what is taught in the Goldberg et al system, it would be useful for the display to maintain a uniform time base so that differences in pulse rate will be readily discernable.
There is additional advantage in being able to synchronize the pulses on successive tracks of the helical trace produced on the drum. An ECG signal related to a heart beat contains a plurality of "waves": P, QRS, T and U. For diagnostic purposes, cardiologists find it most easy to view the mass of display data when each of a particular wave type on a track of the helical trace is synchronized with corresponding waves on adjacent tracks, e.g., the "R" wave of each heartbeat on each line of the trace lines up with the corresponding "R" waves of heartbeats appearing on the other lines of the trace. A display that is so synchronized allows a cardiologist to readily be apprised of irregularities occurring within and among the various heartbeats recorded.
The system and method of displaying ECG data according to the present invention specifically addresses this synchronization problem. The system continuously records the ECG signal in a helical trace about a cylindrical record using a photographic process, producing a "hard copy" of the ECG trace, easily read by a cardiologist. In a similar fashion to the Goldberg patent, the cylindrical record, once having been completed, is opened into a flat record along a reference or index line thus presenting the cardiologist with a two dimensional picture that may be rapidly scanned for pulses which do not clearly fit within an otherwise regular pattern of recorded pulses.
In the present invention, the cylindrical record turns a fixed angular distance at a predetermined rate of rotation, when triggered by an ECG waveform. An "R" wave detector set to a predetermined threshold, detects the presence of the "R" wave component of a heartbeat and triggers a one shot providing a signal having a predetermined period, suitably 200 msec. The signal from the one shot activates a stepper motor drive providing drive pulses to a stepper motor mechanically coupled to the cylindrical record.
Thus, upon detection of the "R" wave, the record turns for 200 msec. to capture the most significant portion of a heartbeat signal. The record then stops until the next "R" wave again triggers the cylindrical record. To ensure that each "R" wave aligns with the wave in the other lines of the trace, synchronizing means are provided so that the first recorded wave in each successive line of recording begins at a common reference or index line. A position sensor optically senses the presence of the reference or index line to provide a reference for "R" wave triggering on each successive track. With this system, aberrant heartbeats represent flaws in an otherwise regular pattern.
A further improvement of the present invention is the means for recording on the readable trace. The somewhat clumsy pen and ink recording system found in previous devices is eliminated. In its place, a digital, photographic recording process is used.
The photographic recording process includes means for converting the electrical signals at input electrodes into optical signals focused at the surface of the cylindrical record. An analog ECG signal at the input electrodes is sampled at a sufficient rate to insure no loss of signal definition and the samples are converted into digital serial data. The digital serial data is demultiplexed into a plurality of address lines for coupling to a linear array of light emitting diodes (LEDs). The LED array is arranged so that a digital signal on a particular address line will light a predetermined one of the LEDs. Optical fibers couple each of the LEDs to a lens system having a focus at the surface of the record. The lens system is mounted on a screw mechanism so that it is moveable in the axial direction of the cylindrical record. A belt/pulley arrangement driven by the rotation of the record drives the screw mechanism causing the lens system to move in the axial direction of the cylindrical record as the drum rotates. Thus, a helical photographic trace is formed on the record.